1. Field of the Invention
The present invention relates to a process for the separation of acrylic acid from its solutions in tri-n-butyl phosphate, and, more especially, relates to such process which is carried out utilizing a unique distillation technique.
2. Description of the Prior Art
It is well known to this art that the separation of acrylic acid from its production reaction media, for example, from the raw reaction product of the oxidation of propylene and/or acrolein, is a delicate and vexing problem and numerous processes have to date been proposed to effect such separation. Specifically, a technique for the absorption in gaseous phase or extraction in liquid phase of the acrylic acid by means of different solvents is known. The acrylic acid is then isolated by distillation, or directly esterified in the extraction medium. Compare French Pat. No. 1,558,432 [Dec. 14, 1967] and French Pat. No. 1,452,566 [Nov. 5, 1965].
Among those solvents already employed for the above purpose, tri-n-butyl phosphate is particularly efficient. But separation of the acid from its solutions in this solvent encounters numerous difficulties. In fact, tributyl phosphate is subject to degradation when exposed to elevated temperatures, especially in the presence of acids or of water, so that depletion in acrylic acid of the solutions at a normal or slightly reduced pressure by distillation results in a significant loss of solvent. On the other hand, in order to obtain a good yield of acrylic acid absorbed or extracted with the aid of tributyl phosphate recycled after distillation, it is necessary that the phosphate have an acrylic acid content as low as possible. To avoid the degradation of the solvent and to lower its acrylic acid content as much as possible, it may be envisaged to perform the distillation in a stronger vacuum. However, in this case difficulties are encountered in the recovery of the acrylic acid, because of its relatively high crystallization point (13.5.degree. C.), corresponding to a vapor pressure of but a few mm. of mercury.